Fix scala#4678: Create a single Typed tree to assign multiple type annotations

This is mostly useful for printers as it represents what is witten in the source.
I may also slightly improve performance of later phases.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1767,8 +1767,19 @@ class Typer extends Namer
     if (ctx.mode is Mode.Type)
       assignType(cpy.Annotated(tree)(arg1, annot1), arg1, annot1)
     else {
+      val arg2 = arg1 match {
+        case Typed(arg2, tpt: TypeTree) =>
+          tpt.tpe match {
+            case _: AnnotatedType =>
+              // Avoid creating a Typed tree for each type annotation that is added.
+              // Drop the outer Typed tree and use its type with the addition all annotation.
+              arg2
+            case _ => arg1
+          }
+        case _ => arg1
+      }
       val tpt = TypeTree(AnnotatedType(arg1.tpe.widenIfUnstable, Annotation(annot1)))
-      assignType(cpy.Typed(tree)(arg1, tpt), tpt)
+      assignType(cpy.Typed(tree)(arg2, tpt), tpt)
     }
   }
 

tests/pos/i4678.scala

@@ -0,0 +1,9 @@
+class Foo {
+  val x: Int = (1: @annot1 @annot2 @annot3 @annot4 @annot5)
+}
+
+class annot1 extends scala.annotation.Annotation
+class annot2 extends scala.annotation.Annotation
+class annot3 extends scala.annotation.Annotation
+class annot4 extends scala.annotation.Annotation
+class annot5 extends scala.annotation.Annotation